In recent years, upon a high-speed interface, such as USB or HDMI, having a higher speed, a measure against radiated noise has been more important. Thus, a common-mode noise, which is a cause of this radiated noise, is removed by a common-mode noise filter. For example, this common-mode noise filter employs ceramic material having a low-dielectric constant for the following reason.
The common-mode noise filter has a structure in which two coils are wound around a dielectric body in the same direction. In general, a current flowing in a coil produces a magnetic field causing a braking effect due to a self-induction. The common-mode noise filter blocks a common-mode noise current from passing therein by using the interaction between the two coils. Specifically, common-mode noise signal currents flows in the two coils in the same direction, and generate magnetic fluxes summed and increased, thereby producing an electromotive force due to the self-induction to provide a higher braking effect, thus preventing the common-mode noise currents from passing.
By moving the two coils to be closer to each other, the magnetic fluxes generated in the coils can be summed and increased to provide the high braking effect, thus providing the common-mode noise filter function in a more favorable manner. However, the two coils moved to be closer to each other causes a large stray capacitance between the coils, which causes a resonance, thus undesirably blocking the passage of a signal current. To prevent this, in order to reduce the distance between the two coils and to reduce the stray capacitance between the coils, a dielectric material around which the coils are wound have a lower dielectric constant.
Furthermore, other high-frequency devices and high-speed signal transmission line substrates also require, as an LTCC material having a high signal propagation velocity and providing an efficient signal transmission, such ceramic materials of dielectric material that have a low dielectric constant. However, even these ceramic materials cannot provide a stable characteristic if the dielectric constant varies. In view of the above, suppressing the variation of the dielectric characteristic of the ceramic material has been desired.
Patent Literature 1 discloses a ceramic electronic component in which pores are provided in a ceramic sintered body using inorganic foaming agent in order to reduce the dielectric constant as well as a manufacture method thereof.
However, in the manufacture method disclosed in Patent Literature 1, since an Ag electrode and a ceramic material have significantly-different shrinkage during a firing shrinkage process, delamination (interface delamination) often occurs at the interface between the Ag electrode and the ceramic material. In general, in order to prevent the Ag electrode and the ceramic material from having an internal stress due to the difference in the shrinkage, the shrinkages of these materials is controlled so as to minimize the difference between the shrinkages in the shrinkage-starting temperature and the shrinkage amount. However, the method of using the foaming agent as described above cannot avoid the expansion of the ceramic material due to the foaming of the foaming agent in the firing shrinkage process, consequently reducing the shrinkage of the ceramic material. Thus, the above method cannot avoid a process causing a significant difference in the shrinkage amount between the Ag electrode and the ceramic material. Specifically, the Ag electrode firstly starts shrinking during firing and then the ceramic material starts shrinking. Then, gas caused by the decomposition of the foaming agent causes the ceramic material to expand during the shrinkage. As a result, the Ag electrode cannot follow the firing shrinkage of the ceramic material, which causes a tensile stress at the surface of the Ag electrode. As a result, delamination is caused at the interface between the Ag electrode and the ceramic material.